I had this same problem with my 5C and it happened that my some of the pins on the NAND chip weren't making a good connection. At the time I didn't have the tools to resolder those connections but I was able to cut a piece of a business card and place it in between the lcd/digi/prox flex's and the shield that holds them down. Its been working without a problem ever since. This is pretty hit or miss, if you don't have the tools and experience to solder at that level then this is probably your best and safest bet.
I was having this issue replacing the disk drive fixed but you will also have to move over the daughterboard from the optical drive because its linked to the motherboard. You'll get an error 0x11. Atleast that is what happened for me!